I thought we had "per process hash randomization" from 5.8.1, and nothing I linked earlier describes something new/different

Until 5.18, only some hashes are randomized (with a per-process seed). Starting with 5.18, all hashes will be randomized (with a per-process seed).

From a programmer's perspective, nothing has changed. It means you can't rely on the order in which the keys of a hash a returned by keys (except the documented exception). This is nothing new, yet dmq's change caused many distributions on CPAN to start failing. (The problems tended to be with the tests themselves, not the modules being tested.)